In general, an optical transmission fiber consists of a core, cladding around the core, and an exterior coating. Conventionally, the cladding has a refractive index that is less than a refractive index of the core, thereby confining an optical signal within the core. Similarly, the cladding often includes an exterior coating that has a higher refractive index than the cladding. Thus, the exterior coating allows any light energy that escapes the core to quickly exit the fiber rather than reentering the core or propagating as a cladding mode and interfering with the transmission signal propagating therein.
However, while such optical fibers have long been used, this design is not flawless. For example, in unidirectional applications, most junctions between two or more fibers generate coupling loss. In such conditions, the light energy not properly coupled into the downstream fiber core can be injected into the cladding of either the upstream or downstream fiber. For instance, a portion of the input light energy can be incident on the core/cladding interface at an angle less than the critical angle of incidence, as provided by Snell's Law. Upon such an occurrence, this light energy passes from the core and continues through the interface between the cladding and the coating, because the conventional coating has a higher index of refraction than that of the cladding. This light energy may be absorbed by the coating or any surrounding materials (e.g., cabling and packaging materials) and converted into heat energy. The heat energy can cause localized damage to the optical fiber and surrounding materials, which significantly reduces the operational life of the fiber. This is particularly consequential in high-power applications, such as but not limited to those where the transmission signal has a power above 0.5 W. In many cases, the surrounding material may have very poor thermal conduction characteristics, which compounds the damage caused by the light energy escaping the core, ultimately causing the optical fiber to fail prematurely. Of course, any optical components proximate the junction are also susceptible to the elevated temperatures.
Other applications that encourage such premature optical fiber failure include the use of bulk optics. More specifically, bulk optics packages typically require decoupling a signal from a package input fiber and processing the signal with the bulk optics components within the package. Thereafter, the processed signal is typically re-coupled to an output fiber. However, in such arrangements, coupling losses occur between the bulk optics components and/or the ends of the input/output fibers, such as those attributable to optical misalignment. Again, it is likely that this stray light energy will find its way into the cladding of the input or output fibers (pigtails) and cause the package and/or a localized portion of the optical fiber to overheat and fail prematurely.
Similarly, work-site obstructions, disadvantageous panel configurations and other installation/assembly obstacles may result in an optical transmission fiber to be permanently configured with a severe bend, such as one having a radius or kink smaller than about 10 mm. In such instances, the severe curvature of the fiber may cause signal energy propagating along the core to be injected into the cladding. Again, the escaping light energy is converted to heat upon leaving the cladding, which can overheat a localized portion of the optical transmission fiber, resulting in premature failure.
Accordingly, what is needed in the art is an optical transmission fiber that overcomes the above-discussed problems experienced by conventional optical transmission fibers.